Hearing aid systems with bi-directional communication capability are well known in the art. U.S. Pat. No. 5,991,419 discloses a so-called bilateral hearing instrument that comprises two units for placement in a hearing aid user's left and right ears, respectively. Each instrument comprises an associated transceiver circuit so as to provide bi-directional wireless communication between the instruments. WO 99/43185 discloses a resembling binaural digital hearing aid system adapted to exchange raw or processed digital signals between two hearing aids to allow each aid to perform a processing of its own input signal as well as a simulated processing of the processing performed in the other aid, i.e. the hearing aid that is arranged on the user's reverse side. The simulated processing of reverse side signals is performed to provide a binaural signal processing technique that can restore binaural sound perception by taking into account differences in hearing loss and compensation between the user's two ears. U.S. Pat. No. 5,751,820 discloses an integrated circuit design for bi-directional communication utilising reflective communication technology to obtain low power consumption, thereby making the design suitable for battery operated personal communication systems, such as binaural digital hearing aid systems.
However, while it has been noted in the above-mentioned prior art that a practical binaural hearing aid system must have control of the synchronisation between the ear units, and that U.S. Pat. No. 5,991,419 states that the phase error between the units should correspond to time errors less than 10 μS, there has not been disclosed an adequate wireless synchronisation technology that would actually be capable of providing the required synchronisation between the units or aids.
To perform correct binaural processing of the respective signals of such binaural hearing aid systems it is mandatory to assure that the individual hearing aids or instruments are operating synchronously with respect to each other. In particular, the respective microphone signals must be sampled substantially synchronously to enable e.g. binaural beamforming and off-axis noise cancellation. Time shifts as small as 20-30 μS between sampling instants of the respective microphone signals in the two hearing aids may lead to a perceivable shift in the beam direction. Furthermore, a slowly time varying time shift between the sampling instants of the respective microphone signals, which inevitably will occur if the hearing aids are operated asynchronously, will result in an acoustic beam that appears to drift and focus in alternating directions. An undesirable effect, which certainly will be very annoying for the hearing aid user.
Consequently, in order to provide a practical binaural hearing system it is highly desirable to provide a wireless communication technique that assures synchronised operation between the individual hearing prostheses and which, at the same time, is practical for miniature and low-power battery operated equipment such as hearing prostheses.